1. Field of Art
The present invention relates to a finish hot rolling method for structural steels and, more specifically, to a hot rolling method capable of precision rolling steel bars, wire and rods.
1. Background Art
A known method to finish hot roll structural steels with high dimensional accuracy comprises application of a light area reduction rate of 10% or less at a finish rolling pass to suppress pass spreading. According to xe2x80x9cPrecision Rolling Method of Structural Steelsxe2x80x9d disclosed in Japanese Unexamined Patent Publication No. H4-371301, for example, when finish rolling structural steels by hot rolling, a heavy area reduction rate of 10% or more is applied at a rolling pass before a finish rolling pass and a light area reduction rate below 10% is applied at the finish rolling pass. Further, according to xe2x80x9cContinuous Hot Rolling Method of Long Structural Steelsxe2x80x9d disclosed in Japanese Patent No. 2857279, a very light area reduction rate of 20% or less of a total area reduction rate of all post-finish rolling passes is applied at the final pass of the post-finish rolling. Both the rolling methods for structural steels, disclosed in the above two publications, aim at suppressing abnormal grains by accumulating strain through successive rolling passes. When the distances between roll stands are long or the rolling speed is slow, however, it is difficult to accumulate strain and therefore it is impossible to suppress the occurrence of abnormal grains. In addition, although the proposed methods can suppress the occurrence of large grains, because of the light area. reduction at the finish rolling pass it is difficult to refine crystal grains to the extent that they do not require normalizing or other heat treatments.
Some technologies employ 3-roll mills for finish rolling of structural steels. For example, according to xe2x80x9cSizing Rolling Method of Steel Bars, Wire and Rodsxe2x80x9d disclosed in Japanese Examined Patent Publication No. H3-50601, sizing rolling from a material diameter to the diameter of 85% of the material diameter is conducted using two 3-roll mills. Also, according to xe2x80x9cFree Size Rolling Method of Steel Bars, Wire and Rodsxe2x80x9d disclosed in Japanese Unexamined Patent Publication No. H7-265904, sizing rolling from a material diameter to the diameter of 95% of the material diameter is conducted using three 3-roll mills. Both rolling methods of steel bars, wire and rods disclosed in the above publications do not disclose any rolling method to achieve good dimensional accuracy and to prevent abnormal microstructures at the same time. It is impossible to obtain a target product diameter by the sizing rolling method of the Japanese Examined Patent Publication No. H3-50601 because, according to the method, the arc diameter of a roll caliber for the final finishing pass is larger than the target diameter of the corresponding product. Using the free size rolling method of the Japanese Unexamined Patent Publication No. H7-265904, it is impossible to obtain both dimensional accuracy and uniform microstructures at the same time.
The object of the present invention is to provide a hot rolling method, for structural steels, capable of both enhancing dimensional accuracy and of homogenizing the microstructure.
The finish hot rolling method for structural steels according to the present invention uses a 2-stand 3-roll finishing mill and is characterized in that an area reduction rate of the final finishing roll pass of the mill is 10 to 20% and that the value of an area reduction rate of the final finishing pass divided by the corresponding area-reduction rate of the roll pass preceding the final finishing pass is 0.7 to 1.3.
Using the present invention, it is possible to minimize pass spreading in a rolling pass since it uses a 3-roll rolling method. It is also possible, according to the present invention, to obtain a uniform microstructure, without depending on an accumulated strain, by setting an area reduction rate of the final finishing pass at 10 to 20%. By setting the value of an area reduction rate of the final finishing pass divided by the corresponding area reduction rate of the preceding pass to 0.7 to 1.3, it is possible to apply an area reduction rate as high as 10% or larger at the final finishing pass without deteriorating product dimension accuracy.
In the finish hot rolling method for structural steels described above, it is preferable that the arc radius of a roll caliber of the preceding pass is 1.0 to 1.3 times the arc radius of the corresponding roll caliber of the final finishing pass. This makes high precision finish rolling of steel products viable.
It is also preferable to form a caliber of the final finishing pass such that the arc radius is equal to the target radius of the corresponding product, the central angle of the arc is 90 to 100xc2x0, and a side wall portion at each side of the caliber extends in a straight line from an end of the arc portion to a roll shoulder. This makes it possible to obtain high dimension accuracy even when applying a high area reduction rate of 10% or larger at the final finishing pass. Dimension accuracy is enhanced also by the fact that the portions of rolled products reduced at the preceding pass are reduced again at the final finishing pass.
When finish rolling is done at a steel temperature of 700 to 800xc2x0 C. at the entry side of the finishing mill in the finish hot rolling method for structural steels described above, an austenite crystal grain size number of No. 8 or better under the Japanese Industrial Standard is achieved and normalizing and other heat treatments can be eliminated thereby.
It is also acceptable to provide a 3-roll mill comprising 2 or more stands in front of the finishing mill, apply a total area reduction rate of 30% or more through all the roll stands, and control the steel temperature at the entry side of the finishing mill to 700 to 900xc2x0 C. This makes the crystal grain of the steel material fed to the 2-stand 3-roll finishing mill finer, and precision finish rolling viable, to obtain structural steels having uniform sectional microstructure and refined crystal grains, allowing elimination of normalizing and other heat treatments.